1. Field of the Invention
The present invention relates to a method for concentrating precious metals contained in the leaching residue discharged from a copper hydrometallurgical process, more particularly method for concentrating precious metals by removing pyrite contained in the leaching residue. The method of the present invention for concentrating precious metals is used to treat leaching residue discharged from a copper hydrometallurgical process which recovers copper from copper sulfide ores.
2. Description of the Prior Art
At present, copper is produced worldwide mostly by dry smelting processes which treat a copper sulfide concentrate as the starting material. The copper sulfide concentrate is a concentrate of sulfide mineral produced from a ore containing sulfide mineral e.g., chalcopyrite (CuFeS2), by a physical separation process, e.g., ore floatation. It includes the copper minerals described above, iron sulfide minerals, e.g., pyrite (FeS2) and pyrrhotite (Fe1-xS: x=0 to 0.2), and oxide minerals, e.g., silicate mineral as gangue, although its composition mainly depends on the mine. It also includes zinc, lead, Group 15 metals, e.g., arsenic, antimony and bismuth, Group 16 metals, e.g., selenium and tellurium, and precious metals, mainly present in sulfide minerals.
The copper dry smelting processes are suitable for efficiently treating large quantities of ores. However, they involve disadvantages, e.g., large-size facilities and hence high investment costs are required in consideration of low reaction efficiency by small-size facilities, and recovery of massively produced SO2 gas is essential. Under these situations, the hydrometallurgical processes have been recently studied.
One of the copper hydrometallurgical processes widely adopted on a commercial scale comprises steps of spraying sulfuric acid onto a heaped copper ore containing a copper oxide mineral to leach out copper, treating the leaching liquor by solvent extraction to increase its copper concentration, and electrolysis for copper recovery. This process, however, involves a disadvantage of low productivity, when applied to sulfide ores, which accounts for majority of copper ores, because chalcopyrite, present in the ores at a higher content than other minerals, is leached with sulfuric acid slowly, resulting in low leaching yield. Therefore, it is difficult for this process to realize productivity comparable to that of a dry smelting process.
In order to solve this disadvantage, methods have been proposed to leach chalcopyrite under conditions favorable for its leaching. One of the representative processes comprises steps of oxidizing a copper ore or copper concentrate under pressure in a halide-containing acidic solution of sulfate ion, leaching the resultant oxide, solvent extraction of the cupric ion present in the leaching liquor, and electrolysis for copper recovery (disclosed by, e.g., Patent Document 1). Another process comprises a step of leaching a copper concentrate with a leaching solution (e.g., chlorine bromide ion) capable of forming a halide complex, which is followed by electrolysis for recovering the cuprous ion produced by the leaching at a low oxidation-reduction potential (disclosed by, e.g., Patent Document 2).
The hydrometallurgical process has several advantages over the dry smelting process. For example, it needs a reduced facility size and hence facility investment resulting from lower operating temperature, and provides wider flexibility of production schedules, because it can be shut down in shorter cycles. However, the hydrometallurgical process also has major problems to be solved, e.g., leaching rate to be improved with chalcopyrite, recovery of precious metals, and abatement of waste residue.
For recovery of precious metals, their separation may be difficult when a leaching solution for the hydrometallurgical process is excessively oxidative, because most elements present in the starting material may be oxidized and leached out. Precious metals present in trace quantities, e.g., gold and silver, are present generally at a very low content in the leaching liquor into which they are leached.
Several methods for recovering these precious metals have been proposed, e.g., by adsorption on active carbon and in the form of amalgam (disclosed by, e.g., Patent Document 2). However, they are frequently not advantageous over the dry smelting process, depending on cost associated with active carbon or agent, and effects on environments.
Therefore, precious metals have been concentrated in the leaching residue, which is treated by a traditional smelting route for copper concentrate, in an existing smelting plant, or by a melting furnace for industrial waste disposal containing copper and so on, where it works as a sulfur source (disclosed by, e.g., Patent Document 3). However, these methods are far from optimum with respect to efficiency, because of increased quantities of pyrite, which press the dry step capacity, increased sulfuric acid production cost resulting from sulfur present in the pyrite, increased iron to be disposed as slag, and so on.
Moreover, the leaching residue contains pyrite, precious metals and gangue, in addition to elementary sulfur, and is frequently less stable chemically than slag discharged from a dry smelting process. Therefore, an adequate countermeasure against impurities of the leaching residue is essential. It is preferable to cut down waste residue by enhancing leaching rate of iron, which is the major ingredient of the residue, in the leaching step and treat pyrite present at a high concentration as the countermeasures.
With a copper concentrate as the starting material, however, pyrite accounting for a large share of the resulting residue is not only in itself sparingly soluble in an acid but also difficult to leach while accelerating leaching a coexisting copper sulfide mineral. Precious metals, e.g., gold, are present in a copper concentrate at a very low concentration and distributed in copper and pyrites. Therefore, disposal of pyrite without recovering the precious metals will lead to a major economic loss. It should be noted, however, that complete leaching of pyrite needs a high oxidative power, which causes major process problems, e.g., oxidation of sulfur and leaching of precious metals.
Therefore, the leaching step adopts the conditions under which pyrite is distributed in the leaching residue, in order to control oxidation of precious metals and sulfur. As a result, the leaching residue builds up, when an ore containing pyrite at a high concentration is treated, and precious metals are not concentrated sufficiently.
Under these circumstances, there have been demands for hydrometallurgical processes capable of efficiently treating copper sulfide ores by realizing a method for removing pyrite present in the leaching residue therefrom and concentrating precious metals in the residue.
[Patent Document 1]
JP-A-2001-515145 (pages 1 and 2)
[Patent Document 21]
Japanese Patent No. 2,857,930 (pages 1 to 4)
[Patent Document 3]
JP-A-2000-313924 (pages 1 and 2)